vroy_92
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Leuven, Belgium
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For circuit #1, note that the drain node of M3 is a very low impedance node. It is a form of regulated cascode I believe if the input is Vbn. What is the input node for this circuit?
For circuit 2, note that current through M3 is always constant. If output voltage goes up by v1, then gate of M1 and M3 will also go up by v1. Because the input current is constant, Vgs,M1 is constant. and therefore the source of M1 (which also happens to be the drain of M2) will also go up by v1. Thus, from a small signal point of view the drain voltages of the current mirror pair (M2,M4) follow each other. This makes it a very good current amplifier/buffer.
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